1. Field of the Invention
The present invention relates to a mulching paper sheet. More particularly, the present invention relates to a mulching paper sheet capable of retaining a satisfactory mechanical strength when being used for plant culture.
2. Description of the Related Art
Mulching paper sheets are used to inhibit growth of weeds and to retain water and warmth in soil. Hitherto, plastic films, for example, polyolefin films, were employed as mulching sheets for plant culture. However, conventional mulching sheets are disadvantageous in that they have no bio-decomposition property and therefore, after use, the waste mulching sheets must be recovered. The recovering operation is a great load on farm workers.
Compared with the plastic mulching sheets, mulching paper sheets are capable of rotting and being decomposed in soil.
There have been many proposals for improving the conventional mulching paper sheets having the above-mentioned advantages.
Conventional mulching paper sheets are, however, disadvantageous in that they are rapidly degradated due to the rotting action of microorganisms in the soil and the degradation action of sunlight including ultraviolet-rays and heat rays, and thus are broken before the growing time of the plant ends.
Since the conventional mulching paper sheets have a relatively low elongation, the mulching paper sheets are often broken when opened and extended. Therefore, a specific finishing process, for example, a crepe treatment or a clupack treatment is applied to the conventional mulching paper sheets to enhance the extensibility of the sheets.
For example, Japanese Examined Patent Publication No. 50-17,371 discloses a process in which a germicide, an insecticide and/or a herbicide is added to a crepe-processed natural kraft paper sheet during the paper sheet-forming process. Japanese Unexamined Patent Publication No. 5-103,553 discloses a process in which a crepe-processed natural kraft paper sheet is coated or impregnated with a silicone resin, and then resin is hardened to impart a water-proofing property to the paper sheet. Also, Japanese Unexamined Patent Publication No. 6-62,680 discloses a process in which a paper sheet is coated or impregnated with carbon black as a light-shielding material or with a chitosan or chitosan derivative which promotes a propagation of soil bacteria salutary to agricultural plants and restricts the propagation of soil bacteria harmful to the agricultural plants.
These conventional processed mulching paper sheets are still disadvantageous in that they have a poor resistance to weathering.